This invention relates to an output stage for power amplifiers, in particular of the minimum drop, low tension type, for use with apparatus which do not require a high output current.
Circuits of this general type are known and commercially available, like for instance the one shown diagramatically in FIG. 1.
Said circuit essentially comprises current mirror stages which are arranged substantially in mutual cascade relationship and effective to amplify a signal V.sub.i and to supply it in this amplified form to a load. The circuit includes a first line, located downstream of the input transistor T.sub.10 and formed by the transistor T.sub.2, current mirror including the transistor T.sub.3 and diode D.sub.1, and current mirror including the transistor T.sub.4 and diode D.sub.2. The other line comprises the transistor T.sub.5, current mirror including the transistor T.sub.6 and diode D.sub.8, and the current mirror including the transistor T.sub.7 and diode D.sub.9. A circuit of this type can provide a maximum peak-to-peak range for the output signal which is equal to EQU V.sub.s -V.sub.CE sat 4 -V.sub.CE sat 7
where V.sub.s is the power supply voltage, and V.sub.CE sat is the saturation voltages of the two T.sub.4 and T.sub.7 across the collector and emitter electrodes.
The current mirrors used in this circuit are of the multiplying type, i.e. the ratio of the currents supplied by the transistor to the diode of each circuit is equal to the ratio between the component surface area. In particular, by respectively designating with A.sub.1, A.sub.2, A.sub.3, A.sub.4, A.sub.5, A.sub.6, A.sub.7, A.sub.8 and A.sub.9 and areas of the components D.sub.1, D.sub.2, T.sub.3, T.sub.4, T.sub.5, T.sub.6, T.sub.7, D.sub.8, and D.sub.9, the following output currents are obtained: EQU I.sub.5 =(A.sub.4 /A.sub.2).times.(A.sub.3 /A.sub.1).times.(I.sub.C1 .times.-I.sub.1) EQU V.sub.smin =(A.sub.7 /A.sub.9).times.(A.sub.6 /A.sub.8).times.I.sub.4 =(A.sub.7 /A.sub.9).times.(A.sub.6 /A.sub.8).times.I.sub.1
The prior circuit has a minimum operating voltage of EQU V.sub.smin =2V.sub.BE +2V.sub.CE sat
In fact, in analyzing the prior circuit, it appears that there are lines connecting the power supply to ground whereon, if the circuit can continue to operate, the voltage should not drop below that value. In particular, in analyzing the line including the generator I.sub.2, diodes D.sub.11 and D.sub.12, and generator I.sub.3, it may be seen that if the generators have a minimal voltage drop thereacross equal to V.sub.CE sat, and the diodes have a drop of V.sub.BE, the above specified minimum operating voltage is obtained. Consequently, even if there exist paths requiring a minimum operating voltage below that value, the circuit as a whole cannot operate if the voltage supply drops below the value indicated. From a practical standpoint, this means that the circuit cannot operate where the voltage supply drops below about 1.4 V, taking into account that in general the minimum value for V.sub.BE is approximately in the 0.6 to 0.65 V range, whilst the minimum value of the saturation voltage V.sub.CE sat is about 100 mV. Thus, where the circuit is required to reliably operate down to a minimum voltage supply of about 1 Volt, the circuit shown cannot be used.